1. Field of the Invention
This invention relates to a spark plug for internal combustion engines, and a method of manufacturing the same.
2. Description of the Related Art
A spark plug used for the ignition of an internal combustion engine, for example, a gasoline engine for automobiles, etc., has a structure including an insulator on an outer side of a central electrode, a main metal member on an outer side of the insulator, and an earth electrode forming a spark discharge gap between the earth electrode and central electrode and fixed to the main metal member. Such a spark plug is fixed to a cylinder head of an engine via an attaching screw portion formed on an outer circumferential surface of the main metal member, and then put to use. Since the portion of the electrode which forms a spark discharge gap is exposed to a combustion gaseous mixture during an operation of the engine, the temperature of this portion becomes extremely high.
With an increase in recent years in output from an internal combustion engine used for automobiles, etc., the areas occupied by suction and exhaust valves in a combustion chamber have also increased. This makes it necessary to miniaturize the spark plug used to ignite a gaseous mixture, and the temperature of the interior of a combustion chamber tends to become increasingly higher due to a supercharging unit, such as a turbocharger. In order to secure the lifetime of a spark plug satisfactorily even in such a severe condition of use, it is necessary to design the spark plug so as to provide good heat radiation (reduction of heat). The heat of the spark plug is discharged through various paths. Especially, a path extending from an insulator to a cylinder head via an attaching screw portion of a main metal member has a high heat flow, and plays an important role in securing reliable heat radiation. Under these circumstances, an attempt to improve the heat radiation performance of a spark has been made by further increasing the length (screw reach) of this attaching screw portion. When a screw reach is increased, the length of the insulator provided within the main metal member naturally also increases.
Concerning a main metal member of a spark plug, the demand has increased for the reduction of the sizes of portions other than an attaching screw portion, in particular reduction of the hexagonal portion (tool engaging portion) which projects outward from a position higher than that of the attaching screw portion and which is provided so that a wrench can be engaged therewith. The situation causing this demand will be described below. The space above the cylinder head has lost superfluidity due to employing a direct ignition system in which each ignition coil is fixed directly to an upper portion of a spark plug, and the diameter of the plug hole has decreased due to an increase in the areas occupied by the above-mentioned valves. As a result, the distance between opposite sides of the hexagonal portion necessarily decreases to 14 mm or smaller, and since only a hexagonal portion not smaller than 16 mm could be secured in a related art spark plug, a decrease in the diameter of an insulator is further required.
3. Problems to Be Solved by the Invention
When an impact is exerted on an insulator, in which the reduction of the diameter in the direction crossing the axis of a spark plug and the enlargement in the axial direction of the length thereof were effected as mentioned above, problems such as bending and the like of the insulator become liable to occur. In a spark plug of a general structure, a flange type projecting portion called an expanded diameter portion is provided on an outer circumferential surface of the insulator, and a main metal member is joined with the expanded diameter portion by clamping with the rear end portion of the main metal member directed toward this expanded diameter portion. A valley-like section is formed in the inner circumferential edge of the end surfaces of this flange-like expanded diameter portion extending in the circumferential direction, and this valley-like section tends to receive stress concentration in particular due to a notch effect. Since this expanded diameter portion is formed in a position comparatively close to a hexagonal portion (tool engaging portion) of a main metal member, the expanded diameter portion is liable in particular to be influenced by the reduction in the diameter of the hexagonal portion. Therefore, when excessive torsion is exerted on an insulator via the main metal member during the driving of an attaching screw portion into an engine head, or, when an impact is imparted to a plug head portion during the fixing of a spark plug to the engine head, especially, problems, such as bending and the like of the insulator of a small diameter and a large length become liable to occur due to both the increase in bending moment and a decrease in the cross-sectional area of the insulator.
Therefore, an object of the present invention is to provide a spark plug capable of effectively preventing the insulator from being bent during a spark plug fixing operation or when an impact, etc., is imparted to the spark plug due to external causes other than the spark plug fixing force despite the use of an insulator having a diameter which is reduced in accordance with a decrease in the dimensions of a tool engaging portion thereof.
The above object of the present invention has been achieved by providing a spark plug including a shaft type central electrode 3, a shaft type insulator 2 covering an outer side of the central electrode 3, a main metal member 1 which is formed cylindrically so as to be opened at both ends thereof, and which is disposed on an outer side of the insulator 2, and an earth electrode 4 combined at one end thereof with the main metal member 1 and opposed at the other end thereof to the central electrode 3 so as to form a spark discharge gap g, said spark plug having a center axis O in the axial direction of the insulator 2, wherein:
the main metal member is provided on an outer circumferential surface thereof with an attaching screw portion 7 and a tool engaging portion 1e for turning the screw portion 7 into a threaded hole in an internal combustion engine, the interval between opposite sides xcexa3; of the tool engaging portion 1e being not greater than 14 mm,
the side in the axial direction of the insulator 2 on which the spark discharge gap g is formed is defined as the front side and the opposite side thereof is defined as the rear side, the insulator 2 being provided with a expanded diameter portion 2e positioned in the main metal member 1 and projecting radially outward from the outer circumferential surface of the insulator 2, and an intermediate trunk portion 2g having a cylindrical outer circumferential surface, formed adjacently in the axial direction to the front side of the expanded diameter portion 2e and engaged at its front end with a main metal member-side engaging section 1c formed on an inner circumferential surface of the main metal member 4,
an outer circumferential surface of an inclined portion 2j forms a linear incline in a position connecting the expanded diameter portion 2e and intermediate trunk portion 2g, such that the diameter of the inclined portion 2j decreases from the side of the expanded diameter portion 2e toward the intermediate trunk portion 2g in orthogonal projections parallel to said axis, a built-up portion 2k being formed where the extension of the outer circumferential surface of the inclined portion 2j and that of the intermediate trunk portion 2g cross each other, such that a valley-like space defined by these extensions is filled with the built-up portion 2k. 
Referring to the appended claims and the above description, the reference symbol following the name of each structural element is used in reference to that added to a corresponding part illustrated in the attached drawings (FIG. 1, FIG. 2 and FIG. 3). These reference symbols are added so as to only make the present invention easily understood, and do not limit at all the concept of each structural requirement for the invention.
The present invention also relates to a spark plug in which the length between opposite sides of the tool engaging portion 1e formed on the main metal member 1 is not larger than 14 mm due to the above-mentioned circumstances which cause the diameter of the insulator 2 to be reduced. In the insulator 2 of this spark plug, a valley-like space extending in the circumferential direction thereof is formed between the outer circumferential surface of the inclined portion 2j, adjoining one edge of the expanded diameter portion 2e, and the intermediate trunk portion 2g. When torsion of a wrench occurring during a spark plug installation operation and other impact force occurring during the same operation are imparted to the valley-like portion to cause bending moment to work on the same, stress concentration is liable to occur thereon. However, according to the present invention, the built-up portion 2k is provided where the extension of the outer circumferential surface of the inclined portion 2j and that of the outer circumferential surface of the intermediate trunk portion 2g cross each other, so as to fill the valley-like space therewith. This can prevent the stress concentration from occurring, and enables the bending resistance etc. of the insulator to be improved by a large margin.